German patent DE-44 40 613 C1 discloses a one- or two-dimensional array of demodulation pixels. One pixel contains one single photo site (photo diode or CCD gate), which is connected to one or more light-protected storage sites (realized as CCD pixel or MOS capacitor) by electrical switches (realized as CCD transfer gate or as transistor switch). The photo site integrates charge that is generated by incoming light. After this short-time integration the photo charge is transferred into a storage site by activating a switch. If realized in CCD technology, this charge addition can be performed repetitively. For a demodulation application the integration time is chosen to be much shorter than the period of the modulation signal. Thus, the device can be used to sample the incoming modulated signal fast enough such that no temporal aliasing takes place.
Practical realizations known so far always used the CCD principle to realize the photo site, the electrical switch and the storage sites. To connect one photo gate to several storage gates by more than one transfer gate (electrical switch) always occupies space. With today's technologies, accessing the photo site, for example by four transfer gates, forces to realize relatively large photo gates. Charge transfer from the photo site to the storage site (response/efficiency of the switch) is then relatively poor and slow. Additionally, practice shows that it is very difficult to realize four transfer gates with equal transfer efficiencies. Therefore, current realizations suffer from inhomogeneities between the single switch/storage combinations at practical frequencies needed for time-of-flight (TOF) applications (>1 MHz). These effects lead to the fact that with today's technologies the teaching of DE-44 40 613 C1 can only be used for TOF applications when realized or operated as one-switch-one-storage-site-device and a special operation mode: it has to be operated such that the sampling points are acquired temporally serially rather than in parallel. This is a serious restriction if TOF-measurements have to be performed of fast-changing scenes containing moving objects.
German patent application DE-197 04 496 A1 describes a similar pixel structure consisting of at least two modulation photo gates (MPG) and dedicated accumulation gates (AG). An MPG pair is always operated in a balanced mode (as balanced modulator). The charge carriers are optically generated in the depletion region underneath the MPGs by the incoming modulated light and guided to the accumulation gates by a potential gradient. This potential gradient depends on the control signals applied to the MPGs.
DE-197 04 496 A1 includes a pixel realization with only one MPG pair operated sequentially with two phases relative to the phase of the modulated transmitter and thus enabling the measurement of the received light's time delay. As in practical realization of DE-44 40 613 C1, this serial acquisition of an “in-phase” and “quadrature-phase” signal represents a serious drawback when being used for TOF applications with fast-changing scenes.
Additionally, DE-197 04 496 A1 suggests a realization with four MPGs and four AGs, where always two MPGs build a balanced modulation pair, and both pairs are operated with different phase with respect to each other. In that way, four phase-measurements (sampling points) of the incoming light can be measured in parallel. This access to the light sensitive area from four local different places again, as is the case in DE-44 40 613 C1, results in a non-uniform charge distribution and gives each accumulation gate a different offset, which is complicated to compensate. DE-197 04 496 A1 suggests two different possibilities:    (i) The AGs are realized as CCD gates. Then the charge carriers can be integrated under the AGs and read out by a multiplex structure, for example a CCD, after an integration period.    (ii) Alternatively, instead of integrating the charge the AGs can be realized directly as pn-diodes and the signal can be read out as voltage or current (for example with an APS structure), or these signals directly feed a post-processing structure to measure phase and total intensity.
Such a post-processing APS-structure, however, occupies space on the sensor and will always drastically increase the sensor's pixel size and, hence, decrease its fill factor. Additionally, feeding the generated photocurrent directly to an amplification stage before being integrated, adds additional noise sources to the signal and decreases the structure's performance, especially for low-power optical input signals.
German patent application DE-198 21 974 A1 is based on DE-197 04 496 A1. Here some special dimensions and arrangements of the MPGs are suggested. The MPGs are implemented as long and small stripes with gate widths of magnitude of the illumination wavelength and gate lengths of 10 to 50 times this magnitude. Several parallel MPG-AG-pairs form one pixel element. All MPG-AG pairs within one pixel element are operated with the same balanced demodulation-control signal. All AGs are properly connected to a pair of readout wires, which feeds a post-processing circuit for the generation of sum and difference current. One pixel consists of one or more pixel elements, where each pixel element consists of several pairs of MPGs. If one pixel is realized with several pixel elements, the teaching of DE-198 21 974 A1 intends to operate the pixel elements in different phase relations, in particular with a phase difference of 90° (in-phase and quadrature-phase measurement in different pixel-elements). Additionally, DE-198 21 974 A1 recommends the use of microlenses or stripe-lenses to focus the light only onto the (light sensitive) MPGs. These optical structures, however, do not correct for local inhomogeneities in the scene detail imaged to one pixel. Such inhomogeneities, especially to be expected due to the large pixel size, lead to measurement errors. This is because the in-phase pixel elements acquire another part of the scene than the quadrature-phase pixel elements.
The main drawback of DE-198 21 974 A1 is the targeted (relatively large) pixel size between 50×50 μm2 to 500×500 μm2. It is therefore not suited to be realized as a larger array of many 10,000 of pixels. The reason for the described long and narrow MPGs is the need for small transportation distances of the photo-generated charge carriers into the AGs. Only for small distances, the structure can be used for demodulation of high modulation frequencies (increased bandwidth). If the MPGs were realized with MPGs of small length and width, the photosensitive area of each pixel would be very small with respect to the planned space-consuming in-pixel post-processing circuitry. Realizing several long-length and short-width modulation structures and arranging and operating them in parallel and thus increasing the photosensitive area without losing bandwidth (due to small drift ways for the charge carriers) is an elegant way of increasing the optical fill-factor. However, the increased fill factor can only be realized with larger pixels and hence the total number of pixels, which can be realized in an array, is seriously limited with the device described in this prior-art document.
TOF distance measurement systems always use active illumination of the scene. A modulated light source is normally located near the detector. Since the optical power density on the illuminated object or target decreases with the square of the distance between the illumination source and the target, the received intensity on the detector also decreases with the square of the target distance. That is why the measurement accuracy for targets far away from the sensor is worse than the accuracy for near targets.
Some known TOF systems are operated with square light pulses of constant amplitude during the pulse duration (cf. Schroeder W., Schulze S., “Laserkamera: 3D-Daten, Schnell, Robust, Flexibel”, Daimler-Benz Aerospace: Raumfahrt-Infrasttuktur, 1998). The receiver is realized as or combined with a fast electrical, optical or electro-optical switch mechanism, for example an MOS switch, a photomultiplier (1D) or a microchannel plate (MCP), an image intensifier, or the “in-depth-substrate-shutter-mechanism” of special CCDs (Sankaranarayanan L. et al., “1 GHz CCD Transient Detector”, IEEE ch3075-9191, 1991). Spirig's, Lange's and Schwarte's lock-in or demodulation pixels can also be used for this kind of operation (Spirig T., “Smart CCD/CMOS Based Image Sensors with Programmable, Real-time, Temporal . . . ”, Diss. ETH No. 11993, Zurich, 1997; Lange R et al., “Time-of-flight range imaging with a custom solid-state image sensor”, Proc. SPIE, Vol. 3823, pp. 180–191, Munich, June 1999, Lange R. et al., “Demodulation pixels in CCD and CMOS technologies for time-of-flight ranging”, Proc. SPIE, Vol. 3965A, San Jose, January 2000, Schwarte R, German patent application No. DE-197 04 496 A1.
With the transmission of the light pulse, the switch in the receiver opens. The switch closes with the end of the light pulse. The amount of light integrated in the receiver depends on the overlap of the time window defined by the ON time of the switch and the delayed time window of ON time of the received light pulse. Both ON time of the switch and pulse width are chosen to have the same length. Thus, targets with zero distance receive the full amount of the light pulse, the complete light pulse is integrated. Targets farther away from the light source only integrate a fraction of the light pulse. The system can only measure distances L<Lmax within the propagation range of the light pulse, defined by half the product of pulse width T and light velocity c.
The intensity of back-scattered light decreases with the square of the target's distance to the emitting active illumination source. The prior-art shutter operation leads to an additional distance-dependent attenuation of the integrated received signal:
                                          integrated            ⁢                                                  ⁢            signal                    ∼                                                    (                                                      L                    max                                    -                  L                                )                                            L                2                                      ·                          I              trans                                      ,                            (        I        )            where Itrans represents the transmitted light intensity;
                              L          max                =                                            T              ·              c                        2                    .                                    (        2        )            
These prior-art contents are also summarized in FIGS. 8 and 9.
In order to use this principle for performing a distance measurement, two additional measurements have to be performed: a first additional measurement without any active illumination for measuring and subtracting the background-offset, and a second additional measurement with the active illumination switched on for measuring the amplitude of the back-scattered light.